Echolocation-related reversal of information flow in a cortical vocalization network

The mammalian frontal and auditory cortices are important for vocal behavior. Here, using local-field potential recordings, we demonstrate that the timing and spatial patterns of oscillations in the fronto-auditory network of vocalizing bats (Carollia perspicillata) predict the purpose of vocalization: echolocation or communication. Transfer entropy analyses revealed predominant top-down (frontal-to-auditory cortex) information flow during spontaneous activity and pre-vocal periods. The dynamics of information flow depend on the behavioral role of the vocalization and on the timing relative to vocal onset. We observed the emergence of predominant bottom-up (auditory-to-frontal) information transfer during the post-vocal period specific to echolocation pulse emission, leading to self-directed acoustic feedback. Electrical stimulation of frontal areas selectively enhanced responses to sounds in auditory cortex. These results reveal unique changes in information flow across sensory and frontal cortices, potentially driven by the purpose of the vocalization in a highly vocal mammalian model.

illustrates the average dPTE across 500 repetitions calculated using 50 trials corresponding to echolocation, communication (both pre-and post-vocal), or no-voc related LFP segments. A cell (i, j) in a matrix shows the average dPTE value related to the information flow between channels i and j, which occurs in the i → j direction for dPTE values > 0.5 (red colours), and in the j → i direction for dPTE values < 0.5 (blue colours).          . Fig. 1g).

Connectivity patterns during pre-vocal periods
To quantitatively address the differences in preferential information flow shown in Fig. 2, between were strongest in the  and  ranges, although sparse significant differences occurred also in the  and 1 bands. The strength of preferred FAF → AC directionality of information flow was significantly weaker for pre-vocal echolocation than for no-voc periods in  frequencies (Fig. S5). However, in the  band, FAF → AC connectivity was significantly stronger during pre-vocal echolocation periods. Significant differences in the directionality of information flow between communication and no-voc conditions were rare (Fig. S5).

Connectivity patterns during post-vocal periods
There were major differences in connectivity during post-vocal periods between vocalization conditions (Fig. S6b). Preferential top-down information flow was significantly lower for echolocation calls than for communication vocalizations in  and 1 frequencies, but significantly higher in the 2 band (Fig. S6b, top; p < 10 -4 , |d| > 0.8).
Remarkably, post-vocal preferred directionality of information flow in the  and 1 bands was strongest in the bottom-up direction (AC → FAF) for the echolocation condition.
Similar effects were seen when comparing connectivity patterns obtained from post-vocal echolocation and no-voc periods (Fig. S5c, top). In other words, the post-vocal echolocation condition exhibited the weakest top-down information transfer and the strongest bottom up-information flow in bands  and 1. Top-down 2 causal influences remained strongest when animals vocalized an echolocation pulse, as compared to communication call production or no-voc periods. Within-area changes were observed in the -band in FAF, where preferential superficial-to-deep information transfer was significantly higher for echolocation vocalizations (Fig. S6b, top), while deep-tosuperficial information flow was strongest in post-vocal communication and no-voc related periods (Fig. S6b, S5). Finally, significant differences between post-vocal communication and spontaneous activity were limited to  frequencies, and strongest for no-voc LFPs.
We compared the net information outflow across conditions in each structure for postvocal periods (Fig. S6b, bottom). In the -band, preferred information outflow from the

Supplementary Note 2 Passive listening of high-or low-frequency natural sounds does not explain information flow patterns of active vocalization
Recordings for this study were made mostly from an area of the C. perspicillata's AC whose neurons are specialized for processing echolocation sounds. Therefore, it is sensible to assume that the reversal of preferred directionality of information flow from pre-to post-vocal periods during vocalization could be attributed to strong acoustic feedback originating from an echolocation call, interacting with the tuning of the cortical areas recorded. In the following, we present evidence demonstrating that mere auditory feedback is not sufficient to explain our main results.
In a first step, we quantified frequency tuning in the AC (and FAF, see Methods) and observed the tuning of recorded LFPs did not favour the frequency range of echolocation calls (i.e. > 60 kHz), as it peaked at 20-40 kHz for most recording sites (Fig. S7 shows LFP frequency tuning curves measured with 75 dB SPL, 10 ms tones, across penetrations). Thus, LFP responses in the AC, at least based on frequency tuning alone, would not elicit on average a stronger response and therefore a stronger bottom-up transfer towards the FAF. Note that many recordings were responsive at high frequencies, often exhibiting double-peaked tuning curves (e.g. red trace in Fig. S7a). This type of tuning is common in the auditory systems of C. perspicillata and other bats 1, 2, 3 , potentially facilitating neurons to respond to both echolocation and communication sounds. In the FAF, we did not observe clear frequency tuning based on LFPs.
In a second step, we quantified information transfer dynamics in the FAF-AC network in response to acoustic stimulation. Sounds were a high-frequency frequency-modulated sweep ("HF-FM"; intended to mimic an echolocation pulse) and a natural distress syllable ("distress"; as a representative of a communication utterance). These sounds are illustrated in Fig. S7b, together with simultaneous cortical responses to each from FAF and AC in Fig. S7c. When considering LFPs taken in the period from 0-500 ms after stimulus onset, we observed that information flowed predominantly in the FAF → AC direction for bands , , and regardless of the sound considered (Fig S7d). The patterns of information flow were very similar across the types of acoustic stimuli used, although with some significant differences in the  and  bands (quantified in Fig. S8). Overall, preferential information flow dynamics in these and other frequencies were reminiscent of those observed for no-voc periods, as confirmed by scarce differences between information flow patterns associated to passive stimulation and spontaneous activity (Fig.   S8).
Preferential information flow patterns associated to post-vocal echolocation periods were significantly different than those reported for the passive listening of HF-FM sounds (Fig. S9a). Specifically, for  frequencies, information flow in the FAF → AC direction was stronger in the passive listening condition, but stronger in the AC → FAF direction Altogether, these results demonstrate that acoustic input (as may occur from feedback after call utterance) does not account for the reversal of information transfer in  frequencies when animals produce echolocation calls. Our results also provide evidence for a highly dynamic network, in which information reverses in different manners during vocalization and passive listening.